build: cargo-build-system: Strip store hash from vendor-dir.

* guix/build/cargo-build-system.scm (configure): When copying the
sources into the vendor-dir strip off the hash before the package name.

guix/build/cargo-build-system.scm

@@ -99,7 +99,7 @@ Cargo.toml file present at its root."
   (for-each
     (match-lambda
       ((name . path)
-       (let* ((basepath (basename path))
+       (let* ((basepath (string-drop (basename path) 33))
               (crate-dir (string-append vendor-dir "/" basepath)))
          (and (crate-src? path)
               ;; Gracefully handle duplicate inputs